Self-sensing high performance fiber reinforced geopolymer composites

ABSTRACT

The current invention is a novel addition to the field and comprises a self-sensing high performance fiber reinforced Geopolymer composite (HPFR-GPC) with self-sensing ability. In one or more embodiment, the self-sensing abilities are created by the addition of high performance fibers into a Geopolymer composites. The HPFR-GPC exhibits smart, high performance, energy efficient, and sustainability characteristics including: enhanced tensile ductility, toughness, and strain hardening (including crack width control); improved piezoresistive effects; utilization of industrial by-product; high resistance to acid attacks; and lightweight, low density. When compared to current available embedded or attachable sensors, the current invention offers lower cost, higher durability, and a larger sensing volume.

CROSS REFERENCE TO RELATED APPLICATIONS

The application claims the benefit of and priority to U.S. PatentProvisional Application No. 62/463,783, “Self-sensing High PerformanceFiber Reinforced Geopolymer Composites” filed Feb. 27, 2017.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable.

REFERENCE TO A “SEQUENCE LISTING,” A TABLE, OR A COMPUTER PROGRAM

Not Applicable.

DESCRIPTION OF THE DRAWINGS

The drawings constitute a part of this specification and includeexemplary embodiments of the Self-sensing High Performance FiberReinforced Geopolymer Composites, which may be embodied in variousforms. It is to be understood that in some instances, various aspects ofthe invention may be shown exaggerated or enlarged to facilitate anunderstanding of the invention. Therefore the drawings may not be toscale.

FIG. 1 is a graph that illustrates FIG. 3 Typical stress-strain behaviorof HPFR-GPC.

FIG. 2 is a graph that illustrates a typical self-sensing response(piezoresistive effect) of Geopolymer mixture modified with CNF undercompressive dynamic loading at 10 Hz frequency.

FIG. 3 is a graph that illustrates a typical stress-strain behavior ofreplicates samples of HPFR GPC mixtures.

FIG. 4 is a graph that demonstrates stress at first, last cracks, andmaximum stress as a function of NaOH molarity.

FIG. 5 is a graph that demonstrates strain at first and last cracks, andmaximum stress as a function of NaOH molarity.

FIG. 6 is a graph that demonstrates stress at first and last cracks, andmaximum stress as a function of alkali ratio.

FIG. 7 is a graph that demonstrates strain at first crack, last crack,and strain maximum stress as a function of NaOH/Na2SiO3 ratio.

FIG. 8 is a series of graphs that show tensile stress and resistivity ofHPFR-GPC at 0% CNF.

FIG. 9 is a series of graphs that shows tensile stress and resistivityof HPFR-GPC at 0.1% CNF.

FIG. 10 is a series of graphs that shows tensile stress and resistivityof HPFR-GPC at 0.5% CNF.

FIG. 11 is a graph that depicts tensile stress and resistivity of CNFHPFR-GPC mixtures.

BACKGROUND

Geopolymer materials represent an innovative class of “green”technology. Geopolymer systems rely on industrial by-products tosignificantly reduce the carbon footprint. Portland cement manufacturingrequires an immense carbon footprint and is responsible for 85% ofenergy and 90% of CO₂ emissions of the concrete life cycle. Geopolymertechnology could reduce those CO₂ emissions without economic sacrifice,while converting a potentially hazardous industrial by-product to avalue-added construction material. Geopolymer binders utilizealuminosilicate rich industrial by-products and natural materialsincluding fly ash, rice hull ash, furnace slag, and clays, etc. Suchmaterials are activated using alkaline solutions including sodiumhydroxide/sodium silicate to create 3D polymerized molecular chains andnetwork that ultimately develop hardened binder. The material achieveshigh strength within 24-48 hours of thermal curing resulting in anefficient and economical process and material.

Until recently, there are no extensive utilizations of Geopolymermaterials in infrastructure systems. In the U.S., a Geopolymer-Portlandcement blend has been used for limited purposes such as rapid pavementrepairs and military applications, but has not been optimized forwidespread use. Various researchers have developed fiber reinforcedGeopolymers and have demonstrated that the fiber reinforcement improvesthe mechanical and durability characteristics of Geopolymer composites.For example, Geopolymer concrete modified with glass fiber significantlyincreases the compressive strength over a short period of curing.Similarly, blast furnace slag based Geopolymer concrete reinforced withsteel fibers performs substantially better than the Portland cementconcrete reinforced with same steel fibers.

Further research on the behavior of PVA reinforced fly ash basedgeopolymer composites demonstrates that samples showed a strainhardening behavior with a high tensile ductility. Finally, thedeflection-hardening behavior of Geopolymer modified with PVA and steelfiber has been studied. A fiber reinforce Geopolymer matrix wasdeveloped that exhibits sufficient ductility and formation of multiplecracks under bending loads. Concrete mixtures have also been modifiedusing carbon nanotube (“CNT”), carbon nanofiber (“CNF”), in combinationwith short carbon fibers to improve its mechanical properties. Suchmodifications have incorporated piezoresistive effects thereby, makingit as a sensor for damage detection and structural health monitoring.

In summary, although some prior art has reviewed the effects of fiberreinforced Geopolymers, the application, use, and full understanding ofits capabilities is still in infancy. A more robust network ofnanofibers is needed to apply this technology in wide spreadapplication. Formation of a good network of CNF in the mixture not onlyaugments the piezoresistive characteristics but also improves themechanical properties and fracture resistance. The current inventioncomprises a method for the development of self-sensing high performancefiber reinforced Geopolymer composites using carbon nanofiber (“CNF”)and the resulting composition.

DETAILED DESCRIPTION

The subject matter of the present invention is described withspecificity herein to meet statutory requirements. However, thedescription itself is not intended to necessarily limit the scope ofclaims. Rather, the claimed subject matter might be embodied in otherways to include different steps or combinations of steps similar to theones described in this document, in conjunction with other present orfuture technologies.

Geopolymer technology has the potential to substantially reduce CO₂ inthe infrastructure field. Geopolymer technology converts potentiallyhazardous industrial by-product to a value-added construction material.The current invention is a novel addition to the field and comprises aself-sensing high performance fiber reinforced Geopolymer composite(HPFR-GPC) with self-sensing ability. In one or more embodiment, theself-sensing abilities are created by the addition of high performancefibers into a Geopolymer composites. The HPFR-GPC exhibits smart, highperformance, energy efficient, and sustainability characteristicsincluding: enhanced tensile ductility, toughness, and strain hardening(including crack width control); improved piezoresistive effects;utilization of industrial by-product; high resistance to acid attacks;and lightweight, low density. When compared to current availableembedded or attachable sensors, the current invention offers lower cost,higher durability, and a larger sensing volume.

In one embodiment, the invention comprises a method for the developmentof self-sensing high performance fiber reinforced Geopolymer composites(HPFR-GPC) using carbon nanofiber (“CNF”) and the resulting composition.One suitable application, although there are many, of the presentinvention is for engineered cement composites (“ECC”).

HPFR-GPC comprises a Geopolymer binder, additives, and a conductivefiller. In one or more embodiments, the conductive filler comprisesmicrofibers. Any suitable microfiber may be used, such as PolyvinylAlcohol (“PVA”) fibers. PVA fibers facilitate high ductility, increasestrength, demonstrate good crack control, and improve freeze and thawdurability. In the embodiments that employ PVA fibers, the mixturecontains between 0.5 and 5% PVA fibers. Other suitable microfibersinclude polypropylene fibers.

Besides the use of micro-fibers, other fibers may be used to reinforcethe composite. Thus, in one or more embodiments, the conductive fillercomprises a nanofiber. In one or more embodiments, the nanofibercomprises carbon nanofibers (“CNF”). Due to the lower cost of productionand superb mechanical properties, carbon nanofibers (“CNF”) are suitablefor reinforcement of HPFR-GPC, such as ECC. The high Young's modulus,tensile strength, and thermal conductivity of CNFs increase thestiffness, flexural strength, and Young's Modulus of ECC.

Like all fibers, CNFs must be dispersed well within the matrix to workefficiently. Unfortunately, CNFs are subjected to van der Waals forcesthat cause the CNFs to attract one another and form clumps of CNFs.Thus, the CNFs must be uniformly dispersed within the ECC. This methoddescribed herein ensures that the CNF is dispersed within the Geopolymercomposite to provide the highest stiffness and flexural strength and toavoid clumping.

Infrastructure requires periodic inspections to ensure there are nodeveloped flaws that could lead to catastrophic failures. Although somenon-destructive methods are available, such as C-scan, x-ray, eddycurrent, and coin tapping, each is labor intensive, costly, and/orimpractical. Thus, the current invention comprises a practical way toconduct nondestructive monitoring of structures by incorporatingself-sensing materials.

Geopolymer composites, especially ECCs, are ideal for the use ofself-sensing capabilities because of their electrically conductivenature. When CNFs are well dispersed throughout the matrix, theelectrical resistivity of the material is lowered by the electricallyconductive fiber. This increase in conductivity of the material allowsfor the change in resistivity of the material to be easily measured.Once the material is damaged there is an increase in void spaces. Thesevoid spaces form areas of insulating air that increase the resistance ofthe material. Therefore, an increase in the resistivity of a materialwith CNFs can be associated with damage to the material. In one or moreembodiments, CNF is employed to provide self-sensing capabilities to theGeopolymer composite, such as an ECC, as a conductive filler. However,other piezoresitivie inducing materials may be used. In one or moreembodiments, the percentage of CNF is between 0.05 and 0.8 percent.

In one or more embodiments, the Geopolymer composite is dosed with CNFand PVA. In other embodiments, the composite comprises CNF or PVA. Inother embodiments, the conductive filler comprises microfibers andnanofibers. In yet other embodiments, the composite comprises othertypes of suitable fibers.

Additives may include any suitable material that enhance the desiredcharacteristics of the material such as higher compaction, fastercuring, higher strength, and good thermal insulation. In one or moreembodiments, such additives include: rice husk ash and silica sand. Inone or more embodiments, Class F Fly Ash from silica sand is used. Inone or more embodiments, a chemical activator is also used such asSodium Silicate and Sodium Hydroxide. A suitable embodiment employs analkaline (sodium silicate plus sodium hydroxide) to fly ash ration of0.4 to 0.6, a sand to fly ash ratio of 0 to 0.8, a sodium silicate tosodium hydroxide ration of 0 to 2, and a sodium hydroxide molarity of 8to 12.

In order to create HPFR-GPC, the required quantity of fly ash (based onthe weighted percentages) is placed in an aluminum container. Silicasand is also weighed and then placed on top of the fly ash. The silicasand is much denser than the fly ash and may cause homogenous mixingissues if placed on the bottom. The fly ash and silica sand is then drymixed by any suitable means for one to two minutes. The mixing ensuresthat all clumps are dispersed and that the silica sand and fly ash areadequately mixed. The alkaline solution is prepared by mixing a sodiumsilicate solution with a sodium hydroxide solution based on theappropriate ratios and allowed to cool. Superplasticizer is then addedto the alkaline solution and mixed for a short period of time. Anysuitable superplasticizer as known in the art may be used. Next therequired amount of conductive filler is combined with thesuperplasticizer and alkaline solution and mixed at a high shear ratefor between 5 and 15 minutes with an optimum time of 10 minutes. Thealkaline mixtures are then poured in the fly ash and silicia sandmixture and combined for 1 to 5 minutes to ensure a homogenized mixture.The PVA micro fibers and/or CNF are then slowly added and mixed into theresulting mixture in a medium to high shear rate, which avoids any fiberentanglement. The mixtures are then oven cured to temperatures between40 and 60 degrees Celsius.

EXAMPLE 1

Dogbone specimens were prepared and tested under direct tensile loadingusing Materials testing system (“MTS”). The specimens were constructedusing combination of fly ash, silica sand, Polyvinyl Alcohol (“PVA”)fibers, and carbon nanofibers (“CNF”). The dog bone samples had a lengthof 9 inches and an average thickness of 0.5 inches. The widest portionof the sample was located at the far ends and measured 4 inches, whilethe middle measures 2 inches wide. All samples were oven cured at 60degrees Celsius for two days and tested once cooled on the same day ofdemolding. Dog bone samples were then placed in a custom-made grip toevenly distribute forces and to avoid complications dealing withgripping the samples.

To determine the strain created under direct tension, dog bone specimenswere analyzed using Digital Imaging Correlation technology (“DIC”). Tomeasure the deformation of the sample at different stages of testing,DIC technologies compares images taken by a high precision camera andthen determines the average displacement of the overall area ofinterest. To enhance the ability of the camera to capture these minormovements, the surface of the samples were speckled with black and whitepaint. To ensure accurate values, the center of the dog bone, referredas the middle third portion of the sample, was studied. All tensilesamples were tested using a 22 kip materials testing system (“MTS”)uniaxial loading frame under a displacement rate of 0.025 in/min with areal time data acquisition system. Under dynamic loading, a multitude offrequencies were used. Frequencies used during testing included 5 Hz, 1Hz, 0.5 Hz and 0.1 Hz. For each frequency, a loading range was appliedin a sinusoidal fashion. This loading ranged from 0 lbs. to 40% of themaximum tensile strength of the composite.

With regards to the strain sensing ability or piezoresistivity testingof the material, the four-probe method was used. To prepare the samplesfor testing, first both sides of the samples were sanded down. This wasdone to ensure a flat even surface existed and to ensure that the CNFswere exposed. Next, the four strips of conductive silver paint wascarefully applied. The placement of the paint was held constant with allsamples to ensure consistency. The two inner strips were painted exactly3.54 in (9 cm) apart from one another and the two outer strips werelocated exactly 0.59 in (1.5 cm) from the inner strips. After theconductive paint was allowed to dry, a ⅛ in (0.32 cm) thick copper tapewas applied over the paint. Next a conductive wire was soldered to eachpiece of conductive copper paint. These weirs were then used to connectelectrodes to the sample. A constant current was then applied to thesamples through the outer conductive wires and the initial voltage wasread by the MTS machine from electrodes that were placed on the innerconductive wires.

The tensile load, strain, and voltage of the test sample were recordedusing the MTS machine, an extensometer, and the VIC2D software used withDIC. The tensile stress (σ) was determine using the following equation:

$\begin{matrix}{\sigma = \frac{P}{A}} & (1)\end{matrix}$

Where, P=applied tensile load (lb) and A=cross sectional area at themiddle third of the specimen. The resistance (R) of the material wasdetermined by dividing the voltage (V) by the current (I) as shown inequation 2.

$\begin{matrix}{R = \frac{V}{I}} & (2)\end{matrix}$

Where, Resistivity (ρ) was calculated using equation 3.

$\begin{matrix}{\rho = {R\frac{A}{}}} & (3)\end{matrix}$

Where, l is the distance between the inner probes. Finally, theself-sensing ability of the material can be found by the fractionalchange in the resistivity of the sample under loading (equation 4).

$\begin{matrix}\frac{\rho - \rho_{0}}{\rho_{0}} & (4)\end{matrix}$

Where ρ_(o) is the initial resistivity and ρ is the resistivity at anyload level.

A typical self-sensing response (piezoresistive effect) of Geopolymermixture modified with CNF under compressive dynamic loading at 10 Hzfrequency is shown in FIG. 2. It can be seen from the figure that theresistivity of the materials changes as the stress in the materialvaries. It is observed that as the stress increases the fractionalchange in resistivity also increases.

FIG. 3 illustrates a typical stress-strain behavior of replicate samplesof HPFR GPC mixtures. It is obvious that there is significant variationsamongst the samples of a same mixture. The initial portions of loadingare extremely important to the behavior of the composite. It isunderstood that the stress at first crack greatly effects the amplitudeof future stress levels as sustained by the mixtures. Even though thestrain at this stage does not much effect, it has an impact on themodulus determination. Another factor effecting the failure energy ofthe mixtures is the maximum stress indicating the ability of themixtures to sustain this load. In order to develop HPFR GPC the mixturemust have good ductility with a high strain capacity as well as fail ina non-catastrophic way. Because DIC technology was used to obtain thestrain values during the experiment, it was possible to view themicro-cracks that were formed throughout the samples during testing.Higher crack numbers and a wider dispersion of the micro-cracks indicatehigher strain values. For the failure type to be considered ductile, thestress that occur after the strain level at the last major crack mustgradually decrease. This gradual decrease is directly related to theability of the material to produce more micro-cracks after failure hasoccurred. In summary, the stresses and strains, at first crack, atmaximum stress level, and at last crack were investigated. The fractureenergy to failure as well as the elastic modulus of all the mixtureswere also evaluated.

Several HPFR-GPC mixtures were constructed and tested using alkaline tofly ash ratio of 0.5, fiber content of 1.5%, NaOH/Na2SiO3 ratio of 1with 10M NaOH solution and varying percentage of sand/fly ash ratios of0, 0.4, and 0.8. The results of direct tensile test are shown inTable 1. It can be seen from the data that there is an optimum value forsand content (40%) related to highest first crack stress value. On theother hand, the addition of sand generally increases the maximum stressthat samples can sustain. However, little difference was observed forthe maximum stress values for the mixtures containing sand/fly ash ratioof 0.4 and 0.8.

TABLE 1 Summary of direct tensile test results for varying percentage ofsand/fly ash ratio. Sand/Fly ash Ratio 0 0.4 0.8 Standard StandardStandard Average Deviation Average Deviation Average Deviation Stress atfirst crack (KPa) 811 150 1438 85 891 124 Stress at last crack (KPa)1,209 144 1,467 251 1,778 197 Strain first crack (%) 0.043 0.008 0.0140.003 0.018 0.003 Strain last crack %) 1.8 0.8 0.8 0.3 0.3 0.2 Elasticmodulus (MPa) 2,649 1,335 12,779 2,697 8,363 2,369 Fracture energy(KPa-m/m) 24 4.2 13 3.8 9 2.6

It is a common practice to add of fillers such as aggregate to concretemixtures to decrease the cost of production. This practice does indeeddecrease the cost of the product, but augment the properties of theconcrete composite. Table 1 indicates the addition of sand greatlydecreases the strain values obtained at the first crack and increasesthe elastic modulus of geopolymer composite. At sand to fly ash ratiosof 0.4 (mix 1) and 0.8 (mix 9), the strain values do not greatly vary.The lower strain values may be due to the bond formed by the fly ash,sand and fibers. At lower sand contents, the sand maybe more capable ofreacting with the alkaline and fly ash. The addition of sand may alsohinder the reaction rate and cause the fibers to be ripped out insteadof slipping out. Also, as stated before, the addition of sand maydecrease the percentage of sand that is capable of reacting with themixture, leaving small amounts of unreacted sand in the mixture andforming weak points in the specimen. Just like the strain at first crackresponse, the addition of sand seems to dramatically decrease the straincapacity of the mixture at the last crack. Even so, the strain values atthe last crack are much greater than that of normal concrete, usually,0.02-0.03%. Similarly, the fracture energy of the HPFR GPC mixtures withsand/fly ash of 0.4 was lower than the ones with no sand and higher thanthe ration of 0.8.

EXAMPLE 3

Due to the brittle nature of geopolymers, fibers are added to improvethe strain capacity of the composites. HPFR-GPC mixtures were preparedusing alkaline to fly ash ratio of 0.5, NaOH/Na2SiO3 ratio of 1 and 10MNaOH solution, sand/fly ash ratio of 0.4 and varying percentage of fiber0.75%, 1.5%, and 3%. The results of direct tensile test are shown inTable 2. The addition of fibers greatly increases the stress sustainingability of the mixtures. Mixtures with 0.75% PVA fibers by volumeexhibited brittle nature. A dramatic increase in the first and lastcrack stresses were observed for geopolymer mixtures with 1.5% and 3%fiber contents. This increase is stress can be attributed to the fibersability to disperse the stresses by bridging across micro-cracks andcausing hindrance to crack propagation.

It can be seen from the data in Table 2 that at higher fiber contents(3%) the strain at first and last cracks are much higher than that ofthe mixtures with lower fiber contents. Strain capacity of about 2% wasobtained. This is an expected result, being that the addition of fibersallows for the elongation of the mixture by the production of multiplemicro-cracks throughout the mixture. Higher stress levels in combinationwith higher strain capacity also resulted in higher fracture energy. DICimages conclude that the mixtures containing only 0.75% PVA fibers areextremely brittle. On the other hand, the mixtures containing 3% PVAfibers exhibited dispersed stresses throughout the sample. Mixturescontaining a PVA fiber contents of 3% seem to break in a more reliablefashion and for the most part show the most ductile failure type.

TABLE 2 Summary of direct tensile test results for varying percentage ofPVA fibers. PVA Fiber dosage 0.75% 1.50% 3% Standard Standard StandardAverage Deviation Average Deviation Average Deviation Stress at firstcrack (KPa) 283 132 1,438 85 2,052 274 Stress at last crack (KPa) 279104 1,467 251 2,267 276 Strain first crack (%) 0.02 0.01 0.02 0.004 0.080.014 Strain last crack %) 0.33 0.19 0.80 0.23 2.0 0.48 Elastic modulus(MPa) 2,313 1,088 9,823 1,670 9,369 952 Fracture energy (KPa-m/m) 1.30.4 13 3.8 53 5.3

EXAMPLE 4

Geopolymer composite mixtures were made using 2% PVA fiber content,NaOH/Na2SiO3 ratio of 1 and 10M NaOH solution, sand/fly ash ratio of0.4, with varying alkaline ratios of 0.4, 0.5, and 0.6 and cured at 60degrees Celsius. The results of direct tensile test are shown in Table3. Unlike sand, the alkaline to fly ash ratio does not seem to affectthe stress at first and last cracks very much. However the straincapacity improved. Interestingly, the fracture energy exhibited anoptimum value at 0.5 ratio. The workability also seems to have positiveeffect at this ratio.

TABLE 3 Summary of direct tensile test results for varying alkaline/Flyash ratio. Alkaline/FA ratio 0.40 0.50 0.60 Standard Standard StandardAverage Deviation Average Deviation Average Deviation Stress at firstcrack (KPa) 1243 150 1438 85 1555 140 Stress at last crack (KPa) 1628249 1467 251 1524 142 Strain first crack (%) 0.01 0.0004 0.014 0.0030.013 0.002 Strain last crack %) 0.2 0.03 0.8 0.22 1.0 0.090 Elasticmodulus (MPa) 12039 2360 10252 1826 11418 458 Fracture energy (KPa-m/m)11 0.6 13 3.8 8 2.0

To explore the effect of changing NaOH Molar concentrations on thestress capabilities of the geopolymer composites, the stresses at firstand crack last crack along with the maximum stresses were examined atNaOH molarities of 8 M, 10 M, and 12 M (FIG. 4). Such mixtures werecured at 60° C. and consisted of 2% PVA fiber content, NaOH/Na2SiO3ratio of 1, sand/fly ash ratio of 0.4, and alkali/Fly ash ratio of 0.5.Generally, increasing molar concentrations seem to increase the stresscapabilities. Only a slight increase occurs when the molar concentrationincreases from 10 M to 12 M.

Similarly, increasing the NaOH molar concentration increases the straincapacity in the same manner as that of the stress capacity (FIG. 5). Itcan be seen from the following figure that a dramatic increase of 420%in the strain at first crack occurs when the NaOH concentrationincreases from 8 M to 10 M. Likewise, the strain at last crack and thestrain at maximum stress dramatically increase by 140% and 300%respectfully.

EXAMPLE 5

Geopolymer composite mixtures were constructed using 2% PVA fibercontent, 10M NaOH solution, sand/fly ash ratio of 0.4, alkaline/fly ashratio of 0.5, and varying NaOH/Na2SiO3 ratio of 0, 1, and 2. All suchmixtures were cured at 60° C. The effect of changing NaOH/Na2SiO3 ratioson the stress capabilities of the geopolymer materials are shown in FIG.6. It should be noted that increasing the alkaline to alkaline ratiofrom 0 to 1 increases the last crack stress and maximum stress by 55%and 60% respectfully. As for the first crack stress, no true differenceoccurs between the two.

Regarding its effect on the strain capabilities, FIG. 7 shows that theaverage strain capacity of the material at first crack generallydecreases as the ratio increases. However, it is seen that increasingthe NaOH/Na2SiO3 ratio greatly increases the last crack strain of thematerial. This is contradictory of that of the average strain at firstcrack results. The ratio 1 exhibits the optimum stress-strain behavior.

EXAMPLE 6

In order to evaluate the effect of curing temperatures various mixtureswere prepared at curing temperatures of 40, 60 and 80° C. using 2% PVAfiber content, 10M NaOH solution, sand/fly ash ratio of 0.4,alkaline/fly ash ratio of 0.5, NaOH/Na2SiO3 ratio of 1. As expected,increasing the temperature from 40 to 80 degrees Celsius by incrementsof 20 degrees tends to generally increase the stress capability of thegeopolymer composite. It can be shown from the data in Table 4 thatincreasing the temperature to 60 or 80 degrees Celsius does not greatlyaffect the first and last crack stresses of the material.

TABLE 4 Summary of direct tensile test results for the effect of curingtemperatures. Curing Temperature (° C.) 40 60 80 Standard StandardStandard Average Deviation Average Deviation Average Deviation Stress atfirst crack (KPa) 1,555 213 1,735 112 1,995 197 Stress at last crack(KPa) 1,533 34 2,340 192 2,660 340 Strain first crack (%) 0.04 0.01 0.030.01 0.04 0.01 Strain last crack %) 1.3 0.28 1.4 0.25 1.0 0.3 Elasticmodulus (MPa) 2,251 645 4,890 1,084 5,269 801 Fracture energy (KPa-m/m)27 2 49 6 27 3

On the other hand changing curing temperatures show no real effect onthe strain at first crack. The strain at last crack and the strain atmaximum stress does however seem to be effected by the changing ofcuring temperatures. It is noted that the largest strain values areobtained near curing temperatures of 60 degrees Celsius. Increasing thecuring temperature from 40 to 60° C. increases the modulus nearly twofolds. However only marginal improvements are observed hereafter.Considering the overall stress behavior as well as the fracture energyof the mixtures the curing temperature of 60° C. is adequate.

EXAMPLE 7

Geopolymer composite mixtures were constructed using 2% PVA fibercontent, 10M NaOH solution, sand/fly ash ratio of 0.4, alkaline/fly ashratio of 0.5, NaOH/Na2SiO3 ratio 1, and varying CNF dosages of 0.1%,0.25%, 0.5%, and 0.75%. In order to achieve better workability about1-2% of superplasticizer was also added. The geopolymer mixtures werecured at 60° C. before testing. The addition of CNFs to the geopolymermaterial seems to increase the stresses at first crack and at lastcracks. It can be seen from the data in the Table 5 that the addition of0.10% CNF does not affect the strength capabilities of the geopolymermaterial. However, the addition of higher amounts of CNFs like 0.25%,0.50%, and 0.75% greatly increases the overall stress capabilities ofthe material. The addition of CNFs will greatly increase the stress atfirst crack due to the increased ability of the material to bridge nanoto micro cracks caused by curing and the initial stages of loading. Welldispersed CNFs could disperse the stresses throughout the samples sowell that nano-cracks continuously form and postpone the formation oflarger cracks. This results in low initial strain levels and henceincrease in the elastic modulus of the material. However once the microcracks are formed both CNF and PVA fiber work together in suppressingthe formation and crack propagation thus improving the strain capacitiesas shown by up to 2.6% strain levels for 0.5 and 0.75% CNF dosages.Higher stress levels and strain capacities substantially increases thefracture resistance of the materials as observed by higher fractureenergy values in Table 5. Samples with low CNFs produces specimens withno strength gaining capabilities, low stress and strain values and lowenergy values. DIC images also show that increasing CNF contentsincreases the number of cracks that are formed and proves that CNFs arecapable of bridging cracks after the initial failure of the material.

TABLE 5 Summary of direct tensile test results for various CNF dosagesCNF Dosage (%) 0.10% 0.25% 0.50% 0.75% Standard Standard StandardStandard Average deviation Average deviation Average deviation Averagedeviation Stress at First Crack (KPa) 1,373 225 2,231 355 1,568 2622,361 576 Stress at Last Crack (KPa) 1,326 322 2,525 35 2,948 551 3,254562 Strain at First Crack (%) 0.07 0.01 0.02 0.01 0.03 0.01 0.06 0.01Strain at Last Crack (%) 1.0 0.5 1.6 0.1 2.6 0.6 2.5 0.1 Elastic Modulus(MPa) 3,101 669 4,860 1,217 4,462 885 3,899 507 Fracture Energy(Kpa-mm/mm) 10 1 64 20 66 16 72 9

EXAMPLE 8

The self-sensing ability of the material was determined by the abilityof the material to react to a cyclical loading under a spectrum offrequencies. To accomplish this, it is important to first determine ifthe material can produce a trend that mimics that of the loading.

FIG. 8 illustrates the stress and corresponding resistivity response ofthe HPFR GPC with no CNF. At frequencies of 5 Hz, 1 Hz and 0.5 Hz thevoltage response show a plateauing trend. When the tensile loading ismaximized or minimized, it is seen that the voltage response is alsomaximized or minimized. The general trend of the resistivity does notexactly mimic that of the applied loading. This effect is clearer at afrequency of 0.5 Hz and voltage points between the maximum and minimumloadings are sparse. This indicates that the material is incapable ofsensing the minor changings in loading and is only capable of capturingthe maximum and minimum loading responses.

The addition of 0.10% CNFs per weight of fly ash was also explored (FIG.9). Unlike the piezoresistive response of mixtures with no CNF, theresponse of the 0.1% CNF mixture mimic that of the loading very well. Atthe peak values a gradual change in resistivity occurs instead ofplateau effect as observed in mixtures with no CNF. The CNF created aconductive path and a network of fibers which when stretched caused agradual change in resistivity values. The percent difference between themaximum and minimum values was little above 1%, which corresponded tothe strain level of about 0.02% for loading frequencies of 5, 1 and 0.5Hz.

The examination of the piezoresistive response of mixture at CNFcontents of 0.5% revealed that it does follow the general trend of boththe stress and strain values (FIG. 10). This indicates that the materialcan respond to loading. However, due to the drastic decrease inresistance, the percent changes in resistivity are extremely small. Forsamples with a CNF content of 0.50% it was determined that averagepercent differences in resistivity for frequencies of 5 Hz, 1 Hz, and0.5 Hz, were about 0.25%. This may due to the fact that materials hasbecome very conductive and does not sufficient change in resistivityvalues.

FIG. 11 clearly exhibits the relation of stress with the resistivity ofCNF mixtures under tensile ramp loading. It can be seen that theresistivity increases abruptly with the increase in the stress until thefirst crack and then the rate of increase significantly decreases,afterwards. This is mainly due to very low level of strain until thefirst crack. Once the first crack occur the material starts to takelarge strains and hence more obvious relation with the resistivity couldbe observed. In general, the mixture was able to simulate the tensileramp loading response.

For the purpose of understanding the Self-sensing High Performance FiberReinforced Geopolymer Composites, references are made in the text toexemplary embodiments of a Self-sensing High Performance FiberReinforced Geopolymer Composites, only some of which are describedherein. It should be understood that no limitations on the scope of theinvention are intended by describing these exemplary embodiments. One ofordinary skill in the art will readily appreciate that alternate butfunctionally equivalent components, materials, designs, and equipmentmay be used. The inclusion of additional elements may be deemed readilyapparent and obvious to one of ordinary skill in the art. Specificelements disclosed herein are not to be interpreted as limiting, butrather as a basis for the claims and as a representative basis forteaching one of ordinary skill in the art to employ the presentinvention.

Reference throughout this specification to features, advantages, orsimilar language does not imply that all of the features and advantagesthat may be realized should be or are in any single embodiment. Rather,language referring to the features and advantages is understood to meanthat a specific feature, advantage, or characteristic described inconnection with an embodiment is included in at least one embodiment.Thus, discussion of the features and advantages, and similar language,throughout this specification may, but do not necessarily, refer to thesame embodiment.

Furthermore, the described features, advantages, and characteristics maybe combined in any suitable manner in one or more embodiments. Oneskilled in the relevant art will recognize that the Self-sensing HighPerformance Fiber Reinforced Geopolymer Composites may be practicedwithout one or more of the specific features or advantages of aparticular embodiment. In other instances, additional features andadvantages may be recognized in certain embodiments that may not bepresent in all embodiments.

1. A high performance geopolymer composite comprising: a. a geopolymerbinder; b. a conductive filler; and c. additives.
 2. The highperformance geopolymer composite of claim 1 wherein said geopolymerbinder comprises aluminosilicate rich industrial by-products.
 3. Thehigh performance geopolymer composite of claim 1 wherein said conductivefiller comprises microfibers.
 4. The high performance geopolymercomposite of claim 3 wherein said conductive filler comprises polyvinylalcohol fibers.
 5. The high performance geopolymer composite of claim 1wherein said conductive filler comprises carbon nanofibers.
 6. The highperformance geopolymer composite of claim 1 wherein said conductivefiller comprises microfibers and nanofibers.
 7. The high performancegeopolymer composite of claim 1 wherein said additives are selected fromthe group consisting of rice husk, fly ash, and sand.
 8. The highperformance geopolymer composite of claim 1 wherein said additivescomprise Class F Fly Ash from silica sand.
 9. The high performancegeopolymer composite of claim 1 further comprising a chemical activatorsolution.
 10. The high performance geopolymer composite of claim 9wherein said chemical activator solution comprises Sodium Silicate andSodium Hydroxide.
 11. A high performance geopolymer compositecomprising: a. a geopolymer binder; b. polyvinyl alcohol fibers, whereinsaid polyvinyl alcohol fibers content is between 0.5 and 3%; c. achemical activator solution comprising Sodium Silicate and SodiumHydroxide; d. an additive comprising fly ash, wherein said fly ash tosaid chemical activator solution ratio is 0.5; e. a superplasticizer,wherein said superplasticizer content is between 1 and 2 percent; and f.carbon nanofibers, wherein said carbon nanofibers content is between0.25 and 0.8 percent.
 12. A method for creating high performancegeopolymer composite comprising: a. placing a weighed amount of fly ashin an aluminum container and placing a weighed amount of silica sand ontop of said fly ash in said aluminum container; b. dry mixing saidweighed amount of fly ash and said weight amount of silica sand,creating a fly ash-silica sand mixture; c. prepare an alkaline solutionby mixing a sodium silicate solution with a sodium hydroxide solution;d. combining said alkaline solution with a superplasticizer and mixingsaid alkaline solution-superplasticizer mixture; e. adding a conductivefiller to said combined alkaline solution-superplasticizer mixture andcombining at a high shear rate, creating a filler-alkaline solution; f.pouring said filler-alkaline solution into said fly ash-silica mixture;and combining for one to five minutes, creating a filler mixture; and g.adding polyvinyl alcohol fiber into said filler mixture slowly and ovencuring at an oven curing temperature the resulting mixture.
 13. Themethod of claim 12 wherein said weighed amount of fly ash to saidweighed amount of silica sand ratio is between 0.25 and 1.5.
 14. Themethod of claim 12 wherein said alkaline solution has a molarity of 10.15. The method of claim 12 wherein said conductive filler comprisesnanofibers.
 16. The method of claim 15 wherein said nanofibers comprisescarbon nanofibers.
 17. The method of claim 16 wherein said carbonnanofibers content is 0.5 to 0.8 percent.
 18. The method of claim 12wherein said polyvinyl alcohol fiber content is 0.25 to 3 percent. 19.The method of claim 12 wherein said oven curing temperature is between40 and 80 degrees Celsius.
 20. The method of claim 12 wherein said ovencuring temperature is 60 degrees Celsius.